Oil giant BP P.L.C. has lost its attempt to shift over $15 billion of costs related to the Gulf of Mexico oil spill onto contractor Transocean Ltd., increasing the possibility BP may have to foot the entire $42 billion cleanup bill. A U.S. federal judge on Thursday said BP must uphold a clause in its contract with Transocean that would shield the Switzerland-based driller from compensatory damage claims related to the 2010 disaster. That means London-based BP may have to shoulder alone compensation claims brought by the likes of fishermen and hoteliers whose livelihoods were affected by largest offshore oil spill in U.S. history.